Without limiting the scope of the invention, its background is described in connection with virtual phase charge coupled device (CCD) image sensors and bulk charge modulated device (BCMD) image sensors, as an example.
Heretofore, in this field, the virtual phase CCD was developed to provide a single phase CCD comparable in performance to multiphase CCD's while retaining all the advantages of single level structure. See Hynecek, J., "Virtual Phase Charge Transfer Device", U.S. Pat. No. 4,229,752, issued Oct. 21, 1980; and Hynecek, J., "Virtual Phase Technology: A new Approach to Fabrication of Large-Area CCD's", IEEE Transactions on Electron Devices, Vol. ED-28, No. 5, May 1981, which are incorporated herein by reference. The bulk charge modulated device (BCMD) device was developed to achieve optimal imaging performance in all aspects of image sensing. See Hynecek, J., "Bulk Charge Modulated Transistor Threshold Image Sensor Elements and Method of Making", U.S. Pat. No. 4,901,129, issued Feb. 13, 1990; and Hynecek, J., "BCMD--An Improved Structure for High-density Image Sensors", IEEE Transactions on Electron Devices, Vol. 38, No. 5, May 1991, which are incorporated herein by reference.
Charge coupled devices (CCDs) are well known monolithic semiconductor devices and are used in various applications such as shift registers, imagers, infrared detectors, and memories. A virtual phase CCD device contains a single set of gates and a single clocking bias. The virtual phase CCD device operates on the principle of selectively doping different regions of each cell so that clocking the gate affects only the energy bands in a portion of each cell and drives them from below to above the fixed energy bands in the remainder of each cell. The doped region that shields this remainder of a cell from the effect of the clock bias of the gate voltage is normally called the "virtual gate". The virtual gate is a doped region that is built directly into the silicon surface and is biased at the substrate potential. The virtual phase CCD minimizes the possibility for gate-to-gate shorts encountered in previous CCD technologies, and provides high quantum efficiency, excellent uniformity, low dark current, and blemish free imagery.
The BCMD sensor consists of a buried-channel MOS transistor with a specially designed storage well located under the transistor channel in the silicon bulk. When the device is illuminated, charge accumulates in the well and changes the potential profile of the entire structure. This in turn affects the potential of the MOS transistor channel that carries the current. The resulting new level of the channel potential is then simply sensed as a voltage of the source junction of the transistor when the device is connected as a source follower. The well is then easily emptied by applying a large negative pulse to the gate of the transistor. The BCMD well is emptied in the vertical direction to the substrate, whereas charge is emptied from CCD wells in a lateral direction. The resulting BCMD is an X-Y addressable MOS image sensor that has a high-sensitivity low-noise high-blooming overload capability, no detectable smear, and no image lag.
It is well known that image sensors based on the CCD concept provide high performance imaging with minimum fixed pattern noise. On the other hand X-Y addressed sensors such as charge injection devices (CID) and BCMD devices which sense charge in each photosite without any charge transfer have an advantage that they can be read out nondestructively. The nondestructive readout is necessary in devices which are used in still photography or in auto focussing elements or in exposure control elements where the correct integration time is not known before hand. The nondestructive readout can be used to interrogate the sensing element several times to determine in "real time" if enough charge has accumulated for a "good signal" before the element is read out and reset.